1. Field of the Invention
This invention relates to electrophotographic photosensitive members and more particularly to an electrophotographic photosensitive member containing a low molecular organic photoconductor, therewith being improved in electrophotographic characteristics.
2. Description of the Prior Art
Inorganic photoconductive materials such as selenium, cadmium sulfide, and zinc oxide have so far been known as photoconductors for use in electrophotographic photosensitive members. While having a number of advantages, for instance, chargeability to a suitable potential in the dark, little dissipation of the charge in the dark, and rapid dissipation of the charge on light exposure, these photoconductive materials suffer from various disadvantages as follows: Selenium type photosensitive members readily crystallize under the influence of environmental factors such as temperature, humidity, dust, and pressure; in particular when the ambient temperature exceeds 40.degree. C., the crystallization becomes remarkable, thereby the chargeability being deteriorated and white spots appearing on resulting images. Cadmium sulfide type photosensitive members cannot maintain constant sensitivity under high humidity conditions. Zinc oxide type photosensitive members, which require to be sensitized with coloring matter such as Rose Bengale, a typical sensitizing colorant for this purpose, cannot give constant image quality over a long period of time because the coloring matter will undergo a charge deterioration due to repeated corona charging and will be faded by repeated light exposure.
On the other hand, a variety of organic photoconductive polymers including polyvinylcarbazole have been proposed until now. In spite of the superiority of these polymers to the above inorganic photoconductive materials in film forming property and in lightweight character, the polymers have been scarecely put to practical use because they are still unsatisfactory in film forming property and inferior to the inorganic photoconductive materials in sensitivity, durability, and stability to changes in environmental conditions.
Low-molecular organic photoconductors also have been proposed including hydrazone compounds as disclosed in U.S. Pat. No. 4,150,987 and others, triarylpyrazoline compounds as in U.S. Pat. No. 3,837,851 and others, and 9-styrylanthracene compounds as in Japanese Patent Laid-open Nos. 94828/1976 and 94829/1976. Although combined use of a suitably selected binder with these low molecular organic photoconductors has overcome the deficiency of film forming property which has been a problem in the art of organic photoconductive polymers, the low molecular organic photoconductors are rather unsatisfactory with respect to sensitivity.
In view of the above, a lamination type of photosensitive member has been proposed which has two photosensitive layers functioning separately as a charge generation layer and a charge transport layer. This type of photosensitive member can be improved in sensitivity to visible rays, retention of charge, and surface strength. These photosensitive members are described in U.S. Pat. Nos. 3,837,851, cited above, and 3,871,882.
However, these photosensitive members employing the conventional low molecular organic photoconductor in the charge transport layer still require improvements in that sufficient sensitivity has not yet been obtained and the light area and dark area potentials vary to large extents when the photosensitive members are repeatedly charged and exposed many times.